Raman Spectroscopy for Water Pollution Monitoring

Raman Spectroscopy is a vibrational spectroscopic technique, based on the Raman effect and is used for investigating molecular structures. When light is inelastically scattered by matter, the wavelengths of the scattered photons are different from the incident photons. The wavelength shift (or frequency shift) is related to the molecular vibrations, which are unique to each molecular structure. The change in frequency of scattered photons (termed Raman shift), may be positive or negative. Raman spectroscopy involves detecting and recording these wavelength shifts and using them to analyse what is in the sample. A Raman spectrum will have several peaks, each peak corresponding to a specific mode of molecular vibration of a chemical species. The location of the maximum peak leads to species identification, while its intensity value correlates with species concentration. While the basic technique has evolved significantly and there are a myriad of applications, one high potential area of application of conventional Raman spectroscopy is in water pollution monitoring.

With the synthetic chemical industry constantly introducing new molecules that eventually find their way into surface waters as effluents, the effective surveillance of water pollution has become challenging. The traditional grab sampling and batch analysis from end of pipe or final sink, is not suitable, especially where trace contaminants are involved. It is more effective to have real-time, on-line analysis methods. This case study illustrates the potential of Raman spectrometry for on-line analysis of water pollution, caused by dissolved pharmaceutical drugs, pesticides or salts coming from fertilisers.

This case study considers examples representing three categories of important organic pollutants, namely drugs, pesticides and chemical fertilisers, for in-situ analysis by Raman Spectroscopy. The study on drug related molecules was performed on ethinylestradiol (a hormone used in contraceptives and a drug based on niflumic acid (used in anti-inflammatory agents). Among pesticides glyphosates are common and hence considered for this study. With regard to fertilisers, Nitrates are a recognized source of eutrophication of waterways due to agricultural run-off, hence a nitrate was considered for analysis.

Measurements on the polluted water samples were made with a BWTek iRaman^TM spectrometer. The excitation wavelength was 532 nm, with a 4 cm^-1 spectral resolution. The spectral range covered was 175–4000 cm^-1. Calibration was done and then polluted samples analysed. Using this set-up, in this illustrative case study, the limit for Nitrate detection was 100 mg/l while Glyophosate was detectable down to 10 mg/l. The limit achieved for niflumic acid needs improvement, as the minimal concentration this experimental set-up and method could detect was 2500 mg/l.

Figure 1 shows the Raman spectra of a contraceptive pill whose main component is ethinylestradiol and also of a drug based on niflumic acid. For clarity, only the peaks of the main molecules are marked. Figure 2 shows the Raman spectra of a typical dissolved synthetic fertilizer mixture comprising 150 g/l of Potassium Phosphate and 89 g/l of Sodium Nitrate.

Figure 1: Normalized Raman Spectra of a drug containing Ethinylestradiol and another containing Niflumic acid, dissolved in demineralized ASTM D1193 type IV reagent water.

Figure 2: Normalized Raman Spectra of a synthetic salt mixture comprising 150 g/l of Potassium Phosphate and 89 g/l of Sodium Nitrate dissolved in demineralized ASTM D1193 type IV reagent water.

Reference:

Ivana Durickovic, Mario Marchetti, Raman spectroscopy as a polyvalent alternative for water pollution detection, IET Science, Measurement and Technology, 2014, Vol. 8, Issue 3, pp. 122–128.